Distillate production in a hydrocarbon synthesis process.

ABSTRACT

A wax fraction from a hydrocarbon synthesis process is fractionated in a vacuum distillation column prior to any hydrocracking steps. A straight-run distillation fraction is isolated from the vacuum distillation. A heavy wax fraction from the vacuum distillation process is hydroprocessed, and a hydroprocessed distillate fraction is recovered. The straight-run distillate fraction and the hydroprocessed distillate fraction are combined to make a fraction that boils in the range of diesel fuel.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application 61/230,512, filed Jul. 31, 2009. This application claims priority to and benefits from the foregoing, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to a process for increasing the production of distillate fuels from a hydrocarbon synthesis process.

BACKGROUND OF THE INVENTION

The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source, and alternative sources for developing combustible liquid fuel are desirable. An abundant source is natural gas. The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain appreciable amounts of nitrogen and sulfur and generally do not contain aromatic compounds. Fischer-Tropsch synthesis is a exemplary means for converting syngas to higher molecular weight hydrocarbon products.

Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C₂₀+wax, which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Although some catalysts have been developed with selectivity for longer chain hydrocarbons, the hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable.

It would be advantageous to provide a process for hydroprocessing Fischer-Tropsch wax which minimizes the hydrocracking of hydrocarbons in the distillate fuel range.

SUMMARY OF THE INVENTION

Accordingly, a process is provided for producing a distillate fuel from a hydrocarbon synthesis process, comprising; passing a heavy fraction from a hydrocarbon synthesis process to a vacuum distillation zone and recovering at least a first distillate fuel fraction and a heavy wax fraction; passing the heavy wax fraction in combination with hydrogen to a hydrocracking reaction zone for cracking the carbon chain molecules in the heavy wax fraction to shorter molecules; and recovering a first reaction zone effluent from the hydrocracking reaction zone.

In embodiments, the process further comprises: passing a combination of the first reaction zone effluent and a light fraction from the hydrocarbon synthesis process to a hydrotreating reaction zone; and recovering a second reaction zone effluent from the hydrotreating reaction zone.

In embodiments, the process further comprises passing a portion of the second reaction zone effluent to an atmospheric distillation zone and recovering at least a second distillate fuel fraction.

In embodiments, the process further comprises: combining the first distillate fuel fraction and the second distillate fuel fraction to make a combined distillate fuel.

In embodiments, the process further comprises passing the combined distillate fuel to an hydroisomerization reaction zone for decreasing the cloud point of the combined distillate fuel.

In another embodiment, the process comprises: passing a heavy fraction from a hydrocarbon synthesis process in combination with a hydroprocessed bottoms product to a vacuum distillation zone and recovering at least a first distillate fuel and a heavy wax stream therefrom; passing at least a portion of the heavy wax stream in combination with hydrogen to a hydrocracking reaction zone for cracking the carbon chain molecules in the wax product to shorter molecules, and recovering a first reaction zone effluent; combining a light fraction from the hydrocarbon synthesis process with at least a portion of the hydrocracking reaction zone effluent and passing the combined feed, in further combination with hydrogen, to a hydrotreating reaction zone for hydrotreating the combined feed, and recovering a second reaction zone effluent; passing at least a portion of the second reaction zone effluent to an atmospheric distillation zone and recovering at least a second distillate fuel and the hydroprocessed bottoms product; and combining the first distillate fuel with the second distillate fuel to form a combined distillate fuel.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustrative schematic flow diagram illustrating one embodiment of the invention.

FIG. 2 is an illustrative schematic flow diagram showing a hydroisomerization reaction step.

FIG. 3 is another illustrative schematic flow diagram showing a hydroisomerization reaction step.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a process for hydroprocessing Fischer-Tropsch products. The invention in particular relates to an integrated process for producing liquid fuels from a hydrocarbon stream provided by Fischer-Tropsch synthesis, which in turn involves the initial conversion of a carbonaceous material such as coal, oil shale, tar sands, petroleum liquids or petroleum gases, including natural gas, to syngas and conversion of the syngas to higher molecular weight hydrocarbon products.

In embodiments, the fractions used in the process are produced by separating products from a Fischer-Tropsch process into a light (i.e. condensate) fraction and a heavy (i.e. wax) fraction (or, alternatively, obtaining such fractions from an appropriate source). The heavy-fraction is subjected to vacuum distillation conditions and a distillate fraction and a heavy wax fraction are recovered. The heavy wax fraction is subject in turn to hydrocracking through one or more catalyst beds to reduce the chain length. The products of the hydrocracking, optionally after a hydroisomerization step, are combined with the light fraction. The combined fractions are hydrotreated, and, optionally, hydroisomerized. The heavy fraction is subjected to vacuum distillation conditions in a vacuum distillation zone, such as, for example, a vacuum distillation column. In embodiments, a suitable vacuum distillation column is provisioned with multiple distillate side draws to recover more than one distillate fraction from the column, each fraction characterized by a particular mid-boiling point.

The methods are advantageous for many reasons. The hydrocracking of the mid-distillate fraction in the heavy fraction is minimized, relative to the case where the entire C5+ fraction from a Fischer-Tropsch synthesis is subjected to similar hydrocracking conditions. The recovery of products in the desired C₅-C₂₀ range, for example, mid-distillates, can be enhanced by minimizing the hydrocracking of Fischer-Tropsch products in the C₅-C₂₀ range. Further, by removing the lighter portion from the feed to the hydrocracking reactor, the throughput of heavy hydrocarbons to the reactor is increased. The methods allow for heat exchange between the relatively high temperature hydrocracking products and the relatively cool light fraction. This heat exchange can be used to bring the temperature of the light fraction up to the desired hydrotreatment temperature, and also to cool the hydrocracking products down to the desired hydrotreatment temperature.

In one aspect, the methods reduce the number of reactor vessels required for hydroprocessing in a refinery. The methods also permit hydroprocessing of two product streams using a single hydrogen supply and a single hydrogen recovery system. The methods can also extend the life of the hydrocracking catalyst by minimizing contact of the light fraction with the hydrocracking catalysts.

The syngas feed to the Fischer-Tropsch reaction zone comprises carbon monoxide and hydrogen. Methods and carbonaceous materials used in the preparation of the syngas feed are well known.

The products from Fischer-Tropsch reactions performed in slurry bed reactors generally include a light fraction and a heavy fraction. The light fraction (i.e. the condensate fraction) includes hydrocarbons boiling below about 700° F. (e.g., tail gases through middle distillates), largely in the C₅-C₂₀ range, with decreasing amounts up to about C₃₀. In embodiments, the light liquid reaction product includes hydrocarbons boiling in the range of C5-650° F. The heavy fraction (i.e. the wax fraction) includes hydrocarbons boiling above about 600° F. (e.g., vacuum gas oil through heavy paraffins), largely in the C₂₀+ range, with decreasing amounts down to C₁₀. Both the light fraction and the heavy fraction are substantially paraffinic. The heavy fraction generally comprises greater than 70% normal paraffins, and often greater than 80% normal paraffins. The light fraction comprises paraffinic products with a significant proportion of alcohols and olefins. In some cases, the light fraction may comprise as much as 50%, and even higher, alcohols and olefins.

Light hydrocarbons can include methane, ethane, propane, butane and mixtures thereof. In addition, carbon dioxide, carbon monoxide, ethylene, propylene and butenes may be present.

In embodiments, the light fraction is a fraction in which at least 75% by weight of the components have a boiling point in the range of between 50° F. and 700° F. and which includes predominantly components having carbon numbers in the range of 5 to 20, i.e. C₅-C₂₀. In some such embodiments, the light fraction is a fraction in which at least 85% by weight or at least 90% by weight of the components have a boiling point in the range of between 50° F. and 700° F. In an embodiment, the light fraction includes at least 0.1% by weight of oxygenates.

In embodiments, the heavy fraction is a fraction in which at least 80% by weight of the components have a boiling point higher than 650° F. and which includes predominantly C₂₀+ components. In some such embodiments, a heavy fraction is a fraction in which at least 85% by weight or at least 90% by weight of the components have a boiling point higher than 650° F. In embodiments, the heavy fraction includes at least 80% by weight of paraffins. In embodiments, the heavy fraction includes no more than about 1% by weight of oxygenates.

A 650° F.+ containing product stream is a product stream that includes greater than 75% by weight 650° F.+ material, or greater than 85% by weight 650° F.+ material, or greater than 90% by weight 650° F.+ material as determined by ASTM D2887. The 650° F.− containing product stream is similarly defined.

A paraffin is a hydrocarbon with the formula C_(n)H_(2n+2).

An olefin is a hydrocarbon with at least one carbon-carbon double bond.

An oxygenate is a hydrocarbonaceous compound that includes at least one oxygen atom.

A distillate fuel is a material containing hydrocarbons with boiling points between about 60° and 800° F. The term “distillate” means that typical fuels of this type can be generated from vapor streams from distilling petroleum crude. In contrast, residual fuels cannot be generated from vapor streams by distilling petroleum crude, and are then a non-vaporizable remaining portion. Within the broad category of distillate fuels are specific fuels that include: gasoline, naphtha, jet fuel, diesel fuel, kerosene, aviation gas, fuel oil, and blends thereof.

A diesel fuel is a material suitable for use in diesel engines and conforming to at least one of the following specifications:

-   -   ASTM D 975—“Standard Specification for Diesel Fuel Oils”     -   European Grade CEN 90     -   Japanese Fuel Standards JIS K 2204     -   The United States National Conference on Weights and Measures         (NCWM) 1997 guidelines for premium diesel fuel     -   The United States Engine Manufacturers Association recommended         guideline for premium diesel fuel (FQP-1A)

A jet fuel is a material suitable for use in turbine engines for aircraft or other uses meeting at least one of the following specifications:

-   -   ASTM D1655     -   DEF STAN 91-91/3 (DERD 2494), TURBINE FUEL, AVIATION, KEROSINE         TYPE,     -   JET A-1, NATO CODE: F-35.     -   International Air Transportation Association (IATA) Guidance         Materials for Aviation, 4th edition, March 2000.

Natural Gas

Natural gas is an example of a light hydrocarbon feedstock. In addition to methane, natural gas includes some heavier hydrocarbons (mostly C₂-C₅ paraffins) and other impurities, e.g., mercaptans and other sulfur-containing compounds, carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. Natural gas fields also typically contain a significant amount of C₅+material, which is liquid at ambient conditions.

The methane, and optionally ethane and/or other hydrocarbons, can be isolated and used to generate syngas. Various other impurities can be readily separated. Inert impurities such as nitrogen and helium can be tolerated. The methane in the natural gas can be isolated, for example in a demethanizer, and then de-sulfurized and sent to a syngas generator.

Syngas

Methane (and/or ethane and heavier hydrocarbons) can be sent through a conventional syngas generator to provide synthesis gas. Typically, synthesis gas contains hydrogen and carbon monoxide, and may include minor amounts of carbon dioxide, water, unconverted light hydrocarbon feedstock and various other impurities. The presence of sulfur, nitrogen, halogen, selenium, phosphorus and arsenic contaminants in the syngas is undesirable. For this reason, it is desired to remove sulfur and other contaminants from the feed before performing the Fischer-Tropsch chemistry or other hydrocarbon synthesis. Means for removing these contaminants are well known to those of skill in the art. In an embodiment, ZnO guard beds are used for removing sulfur impurities. Means for removing other contaminants are well known to those of skill in the art.

Fischer-Tropsch Synthesis

Catalysts and conditions for performing Fischer-Tropsch synthesis are well known to those of skill in the art, and are described, for example, in EP0921184A1, the contents of which are hereby incorporated by reference in their entirety. In the Fischer-Tropsch synthesis process, liquid and gaseous hydrocarbons are formed by contacting a synthesis gas (syngas) comprising a mixture of H₂ and CO with a Fischer-Tropsch catalyst under suitable temperature and pressure reactive conditions. The Fischer-Tropsch reaction is typically conducted at temperatures of about from 300° to 700° F. (149 to 371° C.), with embodiments within the range from 400° to 550° F. (204° to 228° C.). The Fischer-Tropsch reaction is typically conducted at pressures of about from 10 to 600 psia, (0.7 to 41 bars) with embodiments within the range of 30 to 300 psia, (2 to 21 bars); and catalyst space velocities of about from 100 to 10,000 cc/g/hr. with embodiments within the range of 300 to 3,000 cc/g/hr.

The products may range from C₁ to C₂₀₀+ with a majority in the C₅-C₁₀₀+ range. The reaction can be conducted in a variety of reactor types for example, fixed bed reactors containing one or more catalyst beds, slurry reactors, fluidized bed reactors, or a combination of different type reactors. Such reaction processes and reactors are well known and documented in the literature. Slurry Fischer-Tropsch processes, which is a exemplary process in the practice of the invention, utilize superior heat (and mass) transfer characteristics for the strongly exothermic synthesis reaction and are able to produce relatively high molecular weight, paraffinic hydrocarbons when using a cobalt catalyst. In a slurry process, a syngas comprising a mixture of H₂ and CO is bubbled up as a third phase through a slurry in a reactor which comprises a particulate Fischer-Tropsch type hydrocarbon synthesis catalyst dispersed and suspended in a slurry liquid comprising hydrocarbon products of the synthesis reaction which are liquid at the reaction conditions. The mole ratio of the hydrogen to the carbon monoxide may broadly range from about 0.5 to 4, but is more typically within the range of from about 0.7 to 2.75 or from about 0.7 to 2.5. A Fischer-Tropsch process is taught in EP0609079, also completed incorporated herein by reference for all purposes.

Suitable Fischer-Tropsch catalysts comprise on or more Group VIII catalytic metals such as Fe, Ni, Co, Ru and Re. Additionally, a suitable catalyst may contain a promoter. In embodiments, the Fischer-Tropsch catalyst comprises effective amounts of cobalt and one or more of Re, Ru, Pt, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material. In general, the amount of cobalt present in the catalyst is between about 1 and about 50 weight percent of the total catalyst composition. The catalysts can also contain basic oxide promoters such as ThO2, La2O3, MgO, and TiO2, promoters such as ZrO2, noble metals (Pt, Pd, Ru, Rh, Os, Ir), coinage metals (Cu, Ag, Au), and other transition metals such as Fe, Mn, Ni, and Re. Support materials including alumina, silica, magnesia and titania or mixtures thereof may be used. In embodiments, supports for cobalt containing catalysts comprise titania. Useful catalysts and their preparation are known and illustrative, but nonlimiting examples may be found, for example, in U.S. Pat. No. 4,568,663.

Product Isolation from Fischer-Tropsch Synthesis

The products from Fischer-Tropsch reactions performed in higher temperature reactors are generally gaseous products that can form a liquid product when a portion of the gaseous product condenses. Depending on the particular conditions, these temperatures can vary significantly, for example, with the gaseous reaction product including products with boiling points up to about 700° F.

When the gaseous reaction product from the Fischer-Tropsch synthesis step is being cooled and various fractions collected, the first fractions collected tend to have higher average molecular weights than subsequent fractions.

The waxy fraction from the Fischer-Tropsch reaction is passed directly to vacuum distillation for recovery of distillate fraction(s) contained therein. Recovering at least one distillate fraction from the wax prior to hydrocracking ensures that the distillates are not cracked to less valued low molecular weight products during the step of hydrocracking the wax.

Additional Hydrocarbon Streams

The light and heavy fractions described above can optionally be combined with hydrocarbons from other streams, for example, streams from petroleum refining. The light fractions can be combined, for example, with similar fractions obtained from the fractional distillation of crude oil. The heavy fractions can be combined, for example, with waxy crude oils, crude oils and/or slack waxes from petroleum deoiling and dewaxing operations.

Optional Treatment of the Light Fraction before Hydrotreatment

The light fraction typically includes a mixture of hydrocarbons, including mono-olefins and alcohols. The mono-olefins are typically present in an amount of at least about 5.0 wt % of the fraction. The alcohols are usually present in an amount typically of at least about 0.5 wt % or more.

The fraction can be transmitted via pipes to a position in the hydroprocessing reactor below the last hydrocracking bed and above or within the hydrotreatment beds at a temperature above about 40° C.

Prior to reaction, the pressurized light fraction may be mixed with a hydrogen-containing gas stream. When the fraction is heated upon combination with the heated hydrocracking stream (“hydrocrackate”), the olefins may form heavy molecular weight products, such as polymers. Adding even a small amount (i.e., less than about 500 SCFB) of hydrogen-containing gas to the fraction before it is heated by the hydrocrackate prevents or minimizes formation of the undesirable heavier molecular weight products.

The source of hydrogen can be virtually any hydrogen-containing gas that does not include significant amounts of impurities that would adversely affect the hydrocracking and hydrotreatment catalysts. In particular, the hydrogen-containing gas includes sufficient amounts of hydrogen to achieve the desired effect, and may include other gases that are not harmful to the formation of desired end products and that do not promote or accelerate fouling of the downstream catalysts and hydrotreatment equipment. Examples of possible hydrogen-containing gases include hydrogen gas and syngas. The hydrogen can be from a hydrogen plant, recycle gas in a hydroprocessing unit and the like. Alternately, the hydrogen-containing gas may be a portion of the hydrogen used for hydrocracking the heavy fraction.

After the hydrogen-containing gas is introduced into the fraction, the fraction can be pre-heated, if necessary, in a heat exchanger. The methods of heating the fractions in the heat exchangers can include any methods known to practitioners in the art. For example, a shell and tube heat exchanger may be used, wherein a heated substance, such as steam or a reaction product from elsewhere in the method, is fed through an outer shell, providing heat to the fraction in an inner tube, thus heating the fraction and cooling the heated substance in the shell. Alternately, the fraction may be heated directly by passing through a heated tube, wherein the heat may be supplied by electricity, combustion, or any other source known to practitioners in the art.

Hydroprocessing Reactors

Hydrocracking generally refers to breaking down the high molecular weight components of hydrocarbon feed to form other, lower molecular weight products. Hydrotreatment hydrogenates double bonds, reduces oxygenates to paraffins, and desulfurizes and denitrifies hydrocarbon feeds. Hydroisomerization converts at least a portion of the linear paraffins into isoparaffins.

In hydrocracking reactions, pressures and temperatures are often close to the limit the reactors can handle. Multiple catalyst beds with intermediate cooling stages are typically used to control the extremely exothermic hydrocracking reaction. Because the reactions are exothermic, the temperature of the reaction mixture increases and the catalyst beds heat up as the mixture passes through the beds and the reactions proceed. In order to limit the temperature rise and control the reaction rate, a quench fluid is introduced between the catalyst beds.

Ideally, there is less than a 100° F. temperature rise in each bed, or less than about 50° F. per bed, with cooling stages used to bring the temperature back to a manageable level. The heated effluent from each bed is mixed with the quench fluid in a suitable mixing device (sometimes referred to as an inter-bed redistributor or a mixer/distributor) to cool the effluent sufficiently so that it can be sent through the next catalyst bed.

Typically, hydrogen gas is used as a quenching fluid. The hydrogen gas is typically introduced at around 150° F. or above, which is extremely cold relative to the reactant temperatures (typically between 650° and 750° F.). When multiple catalyst beds are used, hydrogen and/or other quench fluids can be used in the intermediate cooling stages. After the final hydrocracking bed, a quench with hydrogen gas is not required, since the light fraction is combined with the heated hydrocracking products, which then cools the hydrocracking products.

Reactor internals between the catalysts beds are designed to ensure both a thorough mixing of the reactants with the quench fluid and a good distribution of vapor and liquid flowing to the next catalyst bed. Good distribution of the reactants prevents hot spots and excessive naphtha and gas make, and maximizes catalyst life. This is particularly important where the heavy fraction includes an appreciable amount of olefins, which makes it highly reactive. Poor distribution and mixing can result in non-selective cracking of the wax to light gas. Examples of suitable mixing devices are described, for example, in U.S. Pat. No. 5,837,208, U.S. Pat. No. 5,690,896, U.S. Pat. No. 5,462,719 and U.S. Pat. No. 5,484,578, the contents of which are hereby incorporated by reference. A exemplary mixing device is described in U.S. Pat. No. 5,690,896.

The reactor includes a means for introducing the light fraction below the last hydrocracking bed and above or within the first hydrotreating bed. In embodiments, the fraction is introduced as a liquid rather than a gas, to better absorb heat from the heated hydrocrackate.

In embodiments, the reactor is a downflow reactor that includes at least two catalyst beds, with inter-bed redistributors between the beds. The top bed(s) include a hydrocracking catalyst and, optionally, one or more beds include a dewaxing or hydroisomerization catalyst.

In a first embodiment, a hydrocracking reaction zone and a hydrotreating reaction zone are in a single reactor vessel, with the reactor vessel including beds of the hydrocracking catalyst(s) and a bottom bed or beds that include a hydrotreatment catalyst. In this embodiment, the temperature and or pressure at the hydrotreatment catalyst beds can be, and generally are the same as that in the hydrocracking reactor. In a second embodiment, the hydrocracking reaction zone and the hydrotreating reaction zone are in separate reactor vessels, such that a second reaction zone contains a hydrotreatment catalyst, and the combined fractions are sent to the separate reactor. In this embodiment, the temperature and or pressure of the hydrotreatment reactor can be, and generally are different from that in the hydrocracking reactor.

In one embodiment, the products of the hydrocracking reaction can be removed between beds, with continuing reaction of the remaining stream in subsequent beds. U.S. Pat. No. 3,172,836 discloses a liquid/vapor separation zone located between two catalyst beds for withdrawing a gaseous fraction and a liquid fraction from a first catalyst bed. Such techniques can be used if desired to isolate products. However, further hydrocracking of the product would be expected to be minimal, so product isolation is not required.

The catalysts and conditions for performing hydrocracking, hydroisomerization and hydrotreating reactions are discussed in more detail below.

Hydrocracking

The heavy fractions described above can be hydrocracked using conditions well known to those of skill in the art. In an embodiment, hydrocracking conditions involve passing a feed stream, such as the heavy fraction, through a plurality of hydrocracking catalyst beds under conditions of elevated temperature and/or pressure. The plurality of catalyst beds may function to remove impurities such as any metals and other solids which may be present, and/or to crack or convert the feedstock. Hydrocracking is a process of breaking longer carbon chain molecules into smaller ones. It can be effected by contacting the particular fraction or combination of fractions, with hydrogen in the presence of a suitable hydrocracking catalyst at hydrocracking conditions, including temperatures in the range of about from 600° to 900° F. (316° to 482° C.), or 650° to 850° F. (343 to 454° C.) and pressures in the range about from 200 to 4000 psia (13-272 atm), or 500 to 3000 psia (34-204 atm) using space velocities based on the hydrocarbon feedstock of about 0.1 to 10 hr⁻¹, or 0.25 to 5 hr⁻¹. In general, hydrocracking catalysts include a cracking component and a hydrogenation component on an oxide support material or binder. The cracking component may include an amorphous cracking component and/or a zeolite, such as a Y-type zeolite, an ultrastable Y-type zeolite or a dealuminated zeolite. A suitable amorphous cracking component is silica-alumina.

The hydrogenation component of the catalyst particles is selected from those elements known to provide catalytic hydrogenation activity. At least one metal component selected from the Group VIII (IUPAC notation) elements and/or from the Group VI (IUPAC notation) elements are generally chosen. Group V elements include chromium, molybdenum and tungsten. Group VIII elements include iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. The amount(s) of hydrogenation component(s) in the catalyst suitable range from about 0.5% to about 10% by weight of Group VIII metal component(s) and from about 5% to about 25% by weight of Group VI metal component(s), calculated as metal oxide(s) per 100 parts by weight of total catalyst, where the percentages by weight are based on the weight of the catalyst before sulfiding. The hydrogenation components in the catalyst may be in the oxidic and/or the sulfidic form. If a combination of at least a Group VI and a Group VIII metal component is present as (mixed) oxides, it will be subjected to a sulfiding treatment prior to proper use in hydrocracking Suitably, the catalyst includes one or more components of iron, nickel, cobalt, molybdenum, tungsten, platinum and/or palladium.

The hydrocracking particles used herein may be prepared, for example, by blending or co-mulling active sources of hydrogenation metals with a binder. Examples of suitable binders include silica, alumina, clays, zirconia, titania, magnesia and silica-alumina. Other components, such as phosphorous, may be added as desired to tailor the catalyst particles for a desired application. The blended components are then shaped, such as by extrusion, dried and calcined at temperatures up to 1200° F. (649° C.) to produce the finished catalyst particles. Alternatively, equally suitable methods for preparing the amorphous catalyst particles include preparing oxide binder particles, for example, by extrusion, drying and calcining, followed by depositing the hydrogenation metals on the oxide particles, using methods such as impregnation. The catalyst particles, containing the hydrogenation metals, are then further dried and calcined before use as a hydrocracking catalyst.

Exemplary catalyst systems include one or more of zeolite Y, zeolite ultrastable Y, SAPO-11, SAPO-31, SAPO-37, SAPO-41, ZSM-5, ZSM-11, ZSM-48, and SSZ-32.

Hydroisomerization

In one embodiment, the hydrocracked products and/or the light fraction are hydroisomerized to provide branching, thus lowering the pour point. Catalysts useful for isomerization processes are generally bifunctional catalysts that include a dehydrogenation/hydrogenation component, an acidic component. Exemplary hydroisomerization catalysts used herein are not sulfur sensitive but instead are enhanced by the presence of sulfur.

The hydroisomerization catalyst(s) can be prepared using well known methods, e.g., impregnation with an aqueous salt, incipient wetness technique, followed by drying at about 125°-150° C. for 1-24 hours, calcination at about 300°-500° C. for about 1-6 hours, reduction by treatment with a hydrogen or a hydrogen-containing gas, and, if desired, sulfiding by treatment with a sulfur-containing gas, e.g., H₂S at elevated temperatures. The catalyst will then have about 0.01 to 10 wt % sulfur. The metals can be composited or added to the catalyst either serially, in any order, or by co-impregnation of two or more metals. Additional details regarding the hydroisomerization catalysts are described below.

Dehydrogenation/Hydrogenation Component

Exemplary dehydrogenation/hydrogenation components are selected from the a Group VIII metals, including the Group VIII non-noble metals, or a Group VI metal. Such metals include iron, cobalt, nickel, palladium and platinum and mixtures thereof. The Group VIII metal is usually present in catalytically effective amounts, that is, ranging from 0.5 to 20 wt %. In embodiments, the Group VI metal, e.g., molybdenum, is incorporated into the catalyst in amounts of about 1-20 wt %.

Acidic Component

Examples of suitable acid components include crystalline zeolites, catalyst supports such as halogenated alumina components or silica-alumina components, and amorphous metal oxides. Such paraffin isomerization catalysts are well known in the art. The acid component may be a catalyst support with which the catalytic metal or metals are composited. In embodiments, the acidic component is a zeolite or a silica-alumina support.

Exemplary supports include one or more of silica, alumina, silica-alumina, silica-alumina-phosphates, titania, zirconia, vanadia and other Group III, IV, V or VI oxides, as well as Y sieves, such as ultra stable Y sieves. In embodiments, the supports include silica-alumina where the silica concentration of the bulk support is less than about 50 wt %, or less than about 35 wt %, or in the range of 15-30 wt %. When alumina is used as the support, small amounts of chlorine or fluorine may be incorporated into the support to provide the acid functionality.

An exemplary supported catalyst has surface areas in the range of about 180-400 m2/gm, or 230-350 m2/gm, and a pore volume of 0.3 to 1.0 ml/gm, or 0.35 to 0.75 ml/gm, a bulk density of about 0.5-1.0 g/ml, and a side crushing strength of about 0.8 to 3.5 kg/mm.

The preparation of amorphous silica-alumina microspheres for use as supports is described, for example, in Ryland, Lloyd B., Tamele, M. W., and Wilson, J. N., Cracking Catalysts, Catalysis, Volume VII, Ed. Paul H. Emmett, Reinhold Publishing Corporation, New York, (1960).

In embodiments, dewaxing/hydroisomerization catalysts include SAPO-11, SAPO-31, SAPO-41, SSZ-32 and/or ZSM-5.

Hydrotreatment

During hydrotreating, oxygen, and any sulfur and nitrogen present in the feed is reduced to low levels. Aromatics and olefins are also reduced. Hydrotreating catalysts and reaction conditions are selected to minimize cracking reactions, which reduce the yield of the most desulfided fuel product.

Hydrotreating conditions include a reaction temperature between 400° F.-850° F. (204° C.-454° C.), or 650° F.-800° F. (343° C.-527° C.); a pressure between 200 to 4000 psig (pounds per square inch gauge) (3.5-34.6 MPa), or 1000 to 3000 psig (7.0-20.8 MPa); a feed rate (LHSV) of 0.5 hr.sup. −1 to 20 hr−1 (v/v); and overall hydrogen consumption 300 to 2000 scf per barrel of liquid hydrocarbon feed (53.4-356 m3 H2/m3 feed). The hydrotreating catalyst for the beds will typically be a composite of a Group VI metal or compound thereof, and a Group VIII metal or compound thereof supported on a porous refractory base such as alumina. Examples of hydrotreating catalysts are alumina supported cobalt-molybdenum, nickel sulfide, nickel-tungsten, cobalt-tungsten and nickel-molybdenum. Typically such hydrotreating catalysts are presulfided.

The products from the hydrocracking of the heavy fractions described above are combined with at least a portion of the light fractions and the combined fractions subjected to hydrotreatment conditions.

In one embodiment, the light fraction is introduced into a reactor at a level below the last hydrocracking catalyst bed and above or within the hydrotreatment bed. In this embodiment, the temperature and or pressure of the hydrotreatment bed can be, and generally are the same as that in the hydrocracking bed(s). Redistributors are generally placed between catalyst beds, for redistributing the fluids passing from catalyst bed to catalyst bed, and the fluids added to the redistributor (e.g. a hydrogen containing gas or a liquid stream) from outside the reactor. Redistributors are well known in the art (e.g. U.S. Pat. No. 5,690,896). In another embodiment, the products from the hydrocracking reactor are pumped to a separate hydrotreatment reactor, where they are combined with the light fraction. In this embodiment, the temperature and or pressure of the hydrotreatment reactor can be different from that in the hydrocracking reactor.

Catalysts useful for hydrotreating the combined fractions are well known in the art. See, for example, U.S. Pat. Nos. 4,347,121 and 4,810,357 for general descriptions of hydrotreating catalysts and conditions. Suitable catalysts include noble metals from Group VIIIA, such as platinum or palladium on an alumina or siliceous matrix, and Group VIIIA and Group VIB metals, such as nickel-molybdenum or nickel-tin on an alumina or siliceous matrix. U.S. Pat. No. 3,852,207 describes suitable noble metal catalysts and mild hydrotreating conditions. Other suitable catalysts are described, for example, in U.S. Pat. Nos. 4,157,294 and 3,904,513. The contents of these patents are hereby incorporated by reference.

The non-noble (such as nickel-molybdenum) hydrogenation metal is usually present in the final catalyst composition as an oxide or as a sulfide, when such compounds are readily formed from the particular metal involved. Exemplary non-noble metal catalyst compositions contain in excess of about 5 weight percent, or about 5 to about 40 weight percent, molybdenum and/or tungsten, and at least about 0.5, or about 1 to about 15 weight percent of nickel and/or cobalt determined as the corresponding oxides. The noble metal (such as platinum) catalyst contains in excess of about 0.01 percent metal, or between about 0.1 and about 1.0 percent metal. Combinations of noble metals may also be used, such as mixtures of platinum and palladium.

In an embodiment, the hydrotreatment reactor includes a plurality of catalyst beds, wherein one or more beds may function to remove impurities such as any metals and other solids which may be present, one or more additional beds may function to crack or convert the feedstock, and one or more other beds may function to hydrotreat the oxygenates and olefins in the light and/or heavy fraction.

Method Steps

The heavy fraction produced in a hydrocarbon synthesis process is fractionated to remove distillates contained therein. In one embodiment, the heavy fraction is fractionated in an atmospheric distillation column, such that the distillation is conducted at atmospheric (or higher) pressure. In embodiments, the atmospheric distillation zone is maintained at a pressure in the range of greater than 15 psia to 150 psia. In embodiments, the distillate product boils in the range of 300°-800° F. In some such embodiments, the bottoms product from the atmospheric distillation zone is passed to a vacuum distillation zone for further separations. In a separate embodiment, the heavy fraction is fractionated in a vacuum distillation column, such that the distillation is conducted at sub-atmospheric pressure to produce at least one distillate product and a heavy wax stream. In some such embodiments, a vacuum distillation column is operated at pressures in the range of 1 mbar to 15 psia. In embodiments, the distillate product boils in the range of 300°-800° F.

The heavy wax stream is hydrocracked through the beds of the hydrocracking catalyst, with cooling between the beds. After the hydrocracking is complete, the effluent from the last hydrocracking bed is combined with a light fraction from the hydrocarbon synthesis process and the combined fractions subjected to hydrotreatment conditions. In embodiments, the light fraction is a liquid, not a gas at the temperature at which it is combined with the effluent from the hydrocracking beds, so that the liquid adsorbs more heat from the heated effluent.

When the hydrotreatment catalyst is present in one or more beds beneath the beds of hydrocracking catalyst, the light fraction can be added above or within the bed. When the hydrotreatment catalyst is present in a separate reactor, the effluent from the last hydrocracking bed can be combined with the light fraction and then sent to the hydrotreatment reactor.

In embodiments, the products from the hydrotreatment reaction are separated into at least two fractions, a distillate fraction and a hydroprocessed bottoms fraction. The distillate fraction can be subject to distillation, hydroisomerization and/or various additional method steps to provide gasoline, diesel fuel, jet fuel and the like, as known to practitioners in the art.

In embodiments, the products from the hydrotreatment are subjected to hydroisomerization. In some such embodiments, the products from the hydroisomerization reaction are separated into at least two fractions, a distillate fraction and a hydroprocessed bottoms fraction. The light fraction can be subject to additional method steps to provide gasoline, diesel fuel, jet fuel and the like, as known to practitioners in the art. The hydroprocessed bottoms fraction can be subject to further distillation in, for example, a vacuum distillation step.

The bottoms fraction can optionally be recycled to the hydroprocessing reactors, to provide an additional light fraction. Alternatively, the fraction can be subject to distillation, hydroisomerization, dewaxing and/or various additional method steps to provide lube base oil stocks, as known to practitioners in the art.

Exemplary hydroisomerization catalysts include SAPO-11, SAPO-31, SAPO-41, SSZ-32, and ZSM-5. Alternatively, or in addition, the fraction can be subjected to solvent dewaxing conditions, as such are known in the art. Such conditions typically involve using solvents such as methylethyl ketone and toluene, where addition of such solvents or solvent mixtures at an appropriate temperature results in the precipitation of wax from the bottoms fraction. The precipitated wax can then be readily removed using means well known to those of skill in the art.

The method described herein will be readily understood by referring to the embodiment in the flow diagram in the accompanying FIG. 1. In FIG. 1, a syngas feed 8 is delivered to a Fischer-Tropsch reaction zone 10 to produce a light fraction 12 and a heavy fraction 16. In embodiments, the light fraction 12 is treated to remove carbon oxides such as CO₂ and CO (not shown). In embodiments, light hydrocarbons 14 that do not contribute to plant product yields may also be removed from the light fraction.

According to embodiments of the present invention, the heavy fraction 16 is passed to heating zone 40 for preheating to a suitable distillation temperature, and from there the heated fraction 44 is passed to vacuum fractionation zone 50, which operates at subatmospheric pressure for recovery of at least a straight-run diesel fraction 52. Exemplary operating temperatures and pressures of vacuum fractionation zone 50 are selected to provide a diesel fraction which boils within the range of from 300°-800° F., or 350°-750° F., or 400° F.-700° F.

A heavy wax stream 56 comprising unconverted and partially converted wax is passed from vacuum fractionation zone 50 to hydrocracking reaction zone 70 or to preheat furnace 60 and further to hydrocracking reaction zone 70. In embodiments, heavy wax stream 56 is combined with hydrogen-rich feedstream 106 prior to preheating. In other embodiments, heavy wax stream 56 is preheated; the preheated liquid material is then combined with hydrogen. In embodiments, heavy wax stream 56 is first heated, for example, in reactor effluent exchangers and then through a furnace 60 before entering the hydrocracking reaction zone 70. The amount of hydrogen-rich feedstream 106 added to the heavy wax stream 56 is sufficient to result in hydrocracking reactions occurring in hydrocracking reaction zone 70 with limited catalyst fouling and with limited or no unselective cracking, such that the formation of C₄-gases during reaction in hydrocracking reaction zone 70 is minimized. The wax feed is partially converted into products in the reactor. The reactor effluent consists of light vaporized hydrocarbons, distillate oils, heavy unconverted oil, and excess hydrogen not consumed in the reaction.

At least a portion of the effluent 72 from the hydrocracking reaction zone 70 is combined with light fraction 12 from the Fischer-Tropsch reaction zone 10 and hydrogen-rich pressurized gas stream 102 and the combination is passed to hydrotreating reaction zone 80, for conversion of one or more of sulfur-containing, nitrogen-containing, oxygen-containing, olefinic and/or aromatic functional groups. Make-up hydrogen 104, comprising greater than 90 mole % H₂, is also added. In embodiments, the hydrotreating reaction zone feed is saturated and oxygenates are removed during hydrotreating. Of course, it will be understood by those skilled in the art that the hydrotreating and hydrocracking processes may be performed in different zones of the same reactor, or they may be done in different reactors.

The effluent 82 from the hydrotreating reaction zone 80 is cooled and depressurized in separation zone 20. In embodiments, 20 is a series of heat exchangers and hot and cold separation zones, some of which are maintained at high pressure and some at low pressure. These separation zones are generally single stage flash separations, though some will include internal features and/or stripping vapor injection to improve the quality of the separation. The separate streams which are produced in separation zone 20 include a hydrogen-rich gas 24, hydrocarbon liquid 22, and, optionally, aqueous stream 26. The aqueous stream 26 is sent to sour water stripping. The hydrocarbon liquid 22 is heated by process streams and sent to the atmospheric fractionation zone 30 of the product recovery section. The hydrogen-rich gas 24 flows into the suction of the recycle gas compressor 100. The recycle compressor delivers the recycle gas to the hydrocracking reaction zone 70 and hydrotreating reaction zone 80. Part of the hydrogen-rich pressurized gas stream 102 is routed to the hydrocracking reaction zone as quench to control the reactor temperature. A further part of gas stream 102 is routed to hydrotreating reaction zone 80 through line 84 as quench to control the temperature of the hydrotreating reaction zone. The remaining recycle gas that is not used as quench is combined with make-up hydrogen 104 to become the hydrocracker reactor feed gas.

The product recovery section includes an atmospheric fractionation zone 30 and a vacuum fractionation zone 50. In embodiments, the atmospheric fractionation zone 30 serves to recover, for example, the LPG 32, naphtha 34, and hydroprocessed diesel 36. The hydroprocessed bottoms 41 from the atmospheric fractionation zone 30 are combined with the heavy fraction 16; the combined stream 42 flows to the vacuum fractionation zone 50. The vacuum fractionation zone 50 serves to provide the heavy wax stream 56 and recover straight-run diesel fraction 52 from the fresh wax. The heavy wax stream 56 from the vacuum fractionation zone flows to the hydrocracker as described above.

Optionally, the straight-run diesel fraction 52 recovered from the fresh wax is combined with hydroprocessed diesel 36 recovered from the hydroprocessed effluent. The combined distillate fuel 54 can be further treated to reduce the cloud point for improved cold flow properties. In embodiments, the combined distillate fuel 54 has a normal boiling point range in the range of 300°-800° F. In some such embodiments, the combined distillate fuel has a normal boiling point range in the range of 350°-750° F. In some such embodiments, the combined distillate fuel has a normal boiling point range in the range of 400° F.-700° F.

With further reference to FIG. 2, the combined distillate fuel 54 is passed to hydroisomerization reaction zone 110, along with hydrogen-rich stream 108 and quench stream 114, to lower the pour point and/or cloud point. The isomerized diesel 112 from the hydroisomerization reaction zone 110 is recovered as a suitable diesel blend stock.

With further reference to FIG. 3, the at least a portion of the effluent 72 from the hydrocracking reaction zone 70 is passed, along with hydrogen-rich pressurized gas stream 124 and quench gas stream 126, to hydroisomerization reaction zone 120, to lower the pour point and/or the cloud point of the effluent prior to hydrotreating. At least a portion of isomerized product 122 is combined with hydrogen-rich pressurized gas stream 12 and the combination is passed to hydrotreating reaction zone 80 for additional saturation and oxygen removal.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. 

1. A process for producing a distillate fuel, comprising; a. passing a heavy fraction from a hydrocarbon synthesis process to a vacuum distillation zone and recovering at least a first distillate fuel fraction and a heavy wax fraction; b. passing the heavy wax fraction in combination with hydrogen to a hydrocracking reaction zone for cracking the carbon chain molecules in the heavy wax fraction to shorter molecules; and c. recovering a first reaction zone effluent from the hydrocracking reaction zone.
 2. The process of claim 1, further comprising: a. passing a combination of the first reaction zone effluent and a light fraction from the hydrocarbon synthesis process to a hydrotreating reaction zone; and b. recovering a second reaction zone effluent from the hydrotreating reaction zone.
 3. The process of claim 2, further comprising passing a portion of the second reaction zone effluent to an atmospheric distillation zone and recovering at least a second distillate fuel fraction.
 4. The process of claim 3, further comprising combining the first distillate fuel fraction and the second distillate fuel fraction to make a combined distillate fuel.
 5. The process of claim 4, further comprising passing the combined distillate fuel to a hydroisomerization reaction zone for decreasing the cloud point of the combined distillate fuel.
 6. The process of claim 1, further comprising: a. passing the first reaction zone effluent from the hydrocracking reaction zone to a hydroisomerization reaction zone and producing a hydroisomerization reaction zone effluent; and b. passing at least a portion of the hydroisomerization reaction zone effluent to a hydrotreating reaction zone.
 7. A process for producing a distillate fuel from a hydrocarbon synthesis process, comprising: a. passing a heavy fraction from a hydrocarbon synthesis process in combination with a hydroprocessed bottoms product to a vacuum distillation zone and recovering at least a first distillate fuel and a heavy wax stream therefrom; b. passing at least a portion of the heavy wax stream in combination with hydrogen to a hydrocracking reaction zone for cracking the carbon chain molecules in the wax product to shorter molecules, and recovering a first reaction zone effluent; c. combining a light fraction from the hydrocarbon synthesis process with at least a portion of the first reaction zone effluent and passing the combined feed, in further combination with hydrogen, to a hydrotreating reaction zone for hydrotreating the combined feed, and recovering a second reaction zone effluent; d. passing at least a portion of the second reaction zone effluent to an atmospheric distillation zone and recovering at least a second distillate fuel and the hydroprocessed bottoms product; and e. combining the first distillate fuel with the second distillate fuel to form a combined distillate fuel.
 8. The process of claim 7, wherein the hydrocarbon synthesis process is a Fischer-Tropsch process.
 9. The process of claim 7, wherein the hydrocracking reaction zone contains one or more catalysts, each comprising a cracking component and a hydrogenation component on an oxide support material.
 10. The process of claim 9 wherein the cracking component is selected from the group consisting of silica-alumina, Y-type zeolite, ultrastable Y-type zeolite and dealuminated zeolite.
 11. The process of claim 9, wherein the hydrogenation component comprises at least one metal component selected from the Group VIII elements and/or from the Group VI elements.
 12. The process of claim 11, wherein the Group VIII elements are selected from the group consisting of iron, cobalt, nickel, palladium and platinum.
 13. The process of claim 11, wherein the Group VI elements are selected from the group consisting of chromium, molybdenum and tungsten.
 14. The process of claim 7, wherein the hydrotreating reaction zone contains one or more catalysts, each comprising a composite of a Group VI metal or compound thereof with a Group VIII metal or compound thereof supported on a refractory base.
 15. The process of claim 14, wherein the Group VIII elements are selected from the group consisting of iron, cobalt, nickel, palladium and platinum.
 16. The process of claim 14, wherein the Group VI elements are selected from the group consisting of chromium, molybdenum and tungsten.
 17. The process of claim 14, wherein the refractory base is alumina.
 18. The process of claim 7, wherein the atmospheric distillation zone is maintained at a pressure in the range of greater than 15 psia to 150 psia.
 19. The process of claim 7, wherein the vacuum distillation zone is maintained at a pressure of less than 15 psia.
 20. The process of claim 7, wherein the combined distillate fuel has a normal boiling point range in the range of 350° F. to 750° F.
 21. The process of claim 7, wherein the combined distillate fuel is a diesel fuel.
 22. The process of claim 7, wherein the hydrocracking reaction zone and the hydrotreating reaction zone are in a single reactor vessel.
 23. The process of claim 7, wherein the hydrocracking reaction zone and the hydrotreating reaction zone are in separate reactor vessels.
 24. The process of claim 7, wherein the hydrocracking reaction zone is maintained at a pressure in the range of 200 to 4000 psig and at a temperature in the range of 600° F. to 900° F.
 25. The process of claim 7, wherein the hydrotreating reaction zone is maintained at a pressure in the range of 200 to 4000 psig and at a temperature in the range of 400° F. to 850° F. 